Squashed Suns, Giant Suns, and the Green Flash: What Happens at the Horizon

Watch enough sunsets carefully and you’ll notice the sun behaving strangely in its final minutes. It flattens into an oval. It seems enormous compared to the small white disk it was at noon. Over the ocean it may sprout a stem like a vase, split into horizontal slices, or — if you’re lucky and the air is right — flare green for a second or two as it disappears.

None of this is your imagination, and almost none of it is happening at the sun. The last degree above the horizon is the most optically distorted piece of sky you will ever look through: light from the setting sun travels through roughly 35 to 40 times more air than light from the overhead sun, and that long, dense, layered path bends, compresses, splits, and colors the image in ways that are visible to the naked eye.

The Sun You See Is Already Below the Horizon

Start with the strangest fact of all: when you watch the sun’s lower edge touch the sea horizon, the actual sun — the geometric object — is already entirely below it.

Air bends light. Because the atmosphere gets denser toward the ground, a ray of sunlight arriving at a shallow angle curves downward as it descends, following the Earth’s curvature slightly. Your eye traces the incoming ray straight back and perceives the sun higher in the sky than it geometrically is. Near the horizon, this atmospheric refraction lifts the sun’s image by about 0.5° — roughly 34 minutes of arc under standard conditions, which is slightly more than the sun’s own diameter of about 32 minutes of arc.

So the entire visible disk sitting on the horizon at sunset is a refracted image of a sun that has, geometrically, already set. In effect, refraction gives you a few bonus minutes of sun at both ends of the day — about 2 to 4 minutes each at mid-latitudes, and far more near the poles where the sun crosses the horizon at a shallow angle.

Published sunrise and sunset times already account for this. The standard convention defines sunset as the moment the sun’s upper edge appears to sink below an ideal horizon, assuming that typical 34-arcminute refraction plus the sun’s 16-arcminute radius. That “standard refraction” assumption is also the main reason real-world times can drift from computed ones — a topic we cover in how accurate are sunrise and sunset times. The atmosphere doesn’t always cooperate with the standard model, especially in the very layer that matters most.

Why the Setting Sun Looks Squashed

Refraction doesn’t lift the whole solar disk equally. It increases rapidly as you approach the horizon: the sun’s lower limb, sitting closer to the horizon than the upper limb, gets lifted noticeably more. The bottom of the sun is pushed up toward the top, and the disk visibly compresses in the vertical direction.

The effect is substantial. At the horizon, the difference in refraction across the sun’s half-degree height typically shaves around 5 or 6 arcminutes off its vertical extent — squashing a 32-arcminute circle into an oval roughly 26 to 27 arcminutes tall but still 32 wide. That is about a 20 percent flattening, easily visible to the naked eye (through a proper solar filter, or in the final seconds when the sun is dim enough to glance at safely). The horizontal width is untouched because refraction acts only in the vertical direction.

When the low atmosphere is strongly layered — a cold sea surface under warm air, or vice versa — the flattening can become extreme and uneven, with the sun taking on stepped, banded, or lens-like shapes. Every distortion is a vertical temperature profile made visible.

The Giant Sun at the Horizon Is In Your Head

Here is the twist: the one horizon effect everybody notices — the sun looking huge as it sets behind buildings or hills — is the one that isn’t optical at all.

The sun’s angular size is essentially constant all day, about half a degree. Refraction, as we just saw, makes the horizon sun slightly smaller in the vertical direction, not larger. And atmospheric magnification is not a thing — the air near the horizon compresses images vertically; it never enlarges them.

The giant horizon sun is the same perceptual quirk as the famous moon illusion, and it is related to the Ponzo illusion: when an object of fixed angular size appears near reference points your brain reads as distant — trees, rooftops, a far shoreline — the brain judges it larger than the same object seen high in an empty sky. It is a misjudgment of size driven by a misjudgment of distance context, and it happens entirely in visual processing.

You can prove it to yourself two ways. Hold your little finger at arm’s length: it spans about one degree of sky, comfortably twice the width of the sun, and it covers the “giant” horizon sun exactly as easily as it covers the noon sun. Or photograph the sun at the horizon and at midday with the same lens at the same zoom — the disks in the two frames will be the same size (with the horizon disk a bit squashed vertically). The camera has no expectations to fool.

Mirages: Omega Suns and Sliced Suns

Over water, the low atmosphere often produces true mirages of the setting sun, and they come in two main varieties.

The inferior mirage — the “Etruscan vase” or omega sunset. When the water is warmer than the air just above it, light passing through the warm surface layer bends upward, producing an inverted mirror image below the real one — the same physics as the shimmering “wet road” mirage on summer asphalt. As the sun nears the sea, its lower limb meets its own mirage image rising to greet it, and the two merge into a shape like the Greek letter omega, or a vase with a flared foot. It is a classic sight over warm ocean horizons — from Key West to Hawaii — and it signals exactly the conditions that favor a green flash minutes later.

The mock mirage — the sliced sun. When a temperature inversion sits above the observer — a layer of warm air over cooler air, common off the California coast — light crossing the inversion can be bent so that the sun appears cut into thin horizontal strips. Slices detach from the top of the disk, hover, distort, and vanish as the geometry changes. Where the inferior mirage needs a warm surface, the mock mirage needs a layered sky; watching which one you get tells you the temperature structure over the water better than any forecast.

The Green Flash Is Real

The green flash has the reputation of a sailor’s tall tale, but it is thoroughly documented physics — photographed, filmed, and explained.

Two ingredients combine. First, dispersion: air refracts shorter wavelengths slightly more than longer ones, so the atmosphere lifts the sun’s blue-green image a touch higher than its red image. At the horizon, the sun is effectively a stack of overlapping colored disks, with green on top. (Blue and violet would sit higher still, but the long air path usually scatters them away.) The vertical offset is tiny — a few arcseconds to arcminutes — which is why you don’t see a green rim in ordinary conditions.

Second, mirage magnification. When an inferior or mock mirage is in progress, the mirage optics can vertically stretch the last sliver of the sun’s upper limb. That stretching magnifies the thin green upper edge into something the eye can actually resolve: a green segment, blob, or brief flare at the instant the last piece of the sun disappears. Most green flashes last one to two seconds.

To see one, you want a distant, sharp sea horizon (or a very flat land horizon), clean and haze-free air, and ideally visible mirage distortion — a squashed or omega-shaped sun is a good omen. Watch the very last sliver of the upper limb at the final moment of sunset. And a genuine safety warning: do not stare at the sun while waiting. Even the low sun can dazzle and harm your eyes, and a dazzled retina will miss the flash anyway. Glance away until only the last thin sliver remains, then watch. At sunrise the flash also occurs — arguably in cleaner morning air — but you must know exactly where the sun will break the horizon, since you cannot track it beforehand.

Why the Ocean Sunset Doesn’t Match Your App

Stand on a beach with a sunset time from any calculator and time the actual disappearance, and you will often find a mismatch of a minute or more. Several honest reasons stack up.

Your height above the water lowers the horizon. Computed times assume an observer at sea level with the horizon at eye level. Stand on a 30-foot dune or a hotel balcony and the sea horizon dips below true horizontal, so the sun takes longer to reach it — sunset arrives late by roughly a half minute from modest heights, more from a clifftop. Our article on how elevation and horizon shape change sunrise and sunset works through the numbers.

Refraction refuses to be standard. The 34-arcminute standard value is an average; real refraction at the horizon commonly ranges from below 30 to above 40 arcminutes depending on the temperature profile in the lowest air, and in unusual conditions it strays much further. Since the sun near the horizon takes about two minutes to move its own diameter (at mid-latitudes), a few arcminutes of nonstandard refraction translate directly into tens of seconds on your watch.

The last sliver misbehaves. Waves, swell, mirage strips, and boiling distortion make “the moment the sun disappears” genuinely fuzzy — the final sliver can wink out, reappear on a wave trough, and vanish again.

None of this means the computed time is wrong; it means the computed time answers a precisely defined question about an idealized horizon, and your beach is not idealized.

How Sailors Handled the Sunken Horizon

These effects were serious business for celestial navigation. A navigator measuring the sun’s altitude with a sextant used the visible sea horizon as the reference line — but from a ship’s deck 20 or 30 feet up, that horizon sits measurably below true horizontal. Navigators corrected for this “dip of the horizon” with standard tables: the dip in arcminutes is approximately 1.76 times the square root of eye height in meters, so an observer 9 meters up applied a correction of a bit over 5 arcminutes. They applied a separate refraction correction, largest near the horizon, and were trained to distrust low-altitude sights precisely because horizon refraction is so variable.

Land surveyors leveling across long distances fought the same battle, scheduling observations for times of day when near-ground refraction was most stable. The lesson they learned the hard way is the same one behind every phenomenon in this article: the last half degree above the horizon is beautiful, informative, and not to be trusted for precision. Watch it for the omega suns and the green flash — and give your sunset timing an extra minute or two of grace.